This invention relates to hydraulic control systems, and more particularly, to a hydraulic control system for use as an overrunning load control system for hydraulic jacks.
Many of the directional control valves for high pressure hydraulic systems utilize poppet-type check valves for controlling the flow of fluid to and from a hydraulic jack or jacks. The poppet valves are used to more positively block fluid flow therethrough when such valves are closed, as compared with a conventional sliding spool-type valve. Generally, the speed of extension and retraction of the hydraulic jack is controlled by modulating the fluid flow from the pump to such hydraulic jacks. Although such control valves have functioned generally satisfactorily in most operating situations, one of the difficulties encountered is that of providing a fine control of an overrunning load situation such as that wherein a heavy load supported by the hydraulic jack is being lowered. In such a situation, the opening of the outlet poppet valve must be controlled for controlling the speed of the hydraulic jack.